In response to relatively recent federal and state "clean air" regulations, carburetor limiter caps have been used to restrict carburetor fuel flow metering valve adjustment to prevent the excessive emission of carbon monoxide and unburned hydrocarbons from internal combustion engine. Primarily, limiters have been used with conventional float bowl carburetors to control automobile exhaust emissions. More recently their use has been extended to diaphragm-type carburetors in small engines simply to prevent gross misadjustment of fuel flow. However, future emissions regulations are expected to extend the role of limiter caps to control engine emissions.
Fuel flow within a carburetor is commonly metered during no load or idle engine operation by a first "idle" needle valve and during part or full load operation by a second "main" needle valve. Typically, each valve has a threaded cylindrical body with an enlarged knurled head at one end and a conical or needle shaped valve control surface at the opposite end which is received within an opening in the carburetor body. To calibrate fuel flow, each valve is rotatively adjusted to axially extend or retract the valve control surface within a fuel passage in the carburetor until optimum fuel flow through the passage is achieved. Pre-calibration of the carburetor on a flow test bench may be performed prior to being assembled to an engine. Subsequent overadjustment of fuel flow is prevented by affixing a limiter cap over the head of each valve. After assembly to an engine, it is customary to adjust the metering valves, if needed, to fine tune fuel flow to actual engine demand.
These limiter caps generally consist of a cylindrical body having an opening at one end for axially receiving the valve head in tight fitting engagement to prevent removal and promote rotation of both in unison. A recess in the opposite end of the cap is provided to enable an adjustment tool to engage the valve directly or the cap alone to rotate both in unison to make fuel flow adjustments. Projecting radially outwardly from the cap body is an arm for abutting against a fixed stop extending from the carburetor to limit valve rotation. Representative limiter caps are described in U.S. Pat. No. 3,618,906 Charron and U.S. Pat. No. 5,055,238 Araki.
Stop members such as an integrally cast projection of the carburetor or the body of another cap press-fit on an adjacent needle valve may be used to limit valve and cap rotation and hence adjustment. Similarly, Japanese Patent Publication Jitsuko Sho 61-134555, discloses a hollow cylindrical collar limiter which has an outwardly extending arm for abutting directly against the head of an adjacent valve to limit adjustment.
Previous to the use of these limiting devices, fuel flow metering valves in carburetors were freely adjustable without limitation. At present, however, limiters have not been universally adopted for carburetors for small engine applications. Particularly, because of numerous difficulties, caps of conventional construction have not been used in applications, such as hand-held tools like weed trimmers, edgers, blowers, chain saws, and hedge trimmers, where carburetor access is restricted by an engine shroud enclosing the engine compartment.
While access openings are provided in the shroud by a grommet to adjust each valve, they are not large enough to allow limiter cap installation after final calibration without removing the shroud. Further, limited accessibility adds to the difficulty of accurately orienting the arm of the cap with respect to the optimum valve position and the stop or stops of the carburetor to provide the appropriate range of adjustability. Removal and reorientation of the cap after calibration with the cap secured to the valve is impractical and time consuming as it would require the application of considerable force to remove the limiter possibly damaging the cap. Production time may also be lost if the shroud or grommet must be disassembled to gain sufficient access to the cap. Unfortunately, disturbing the valve in this manner to remove the cap would likely require recalibration before reinstalling the same or a different limiter cap.
For example, if a two-stroke engine is sold with the adjustment range incorrectly set, subsequent user adjustment could increase emissions or deprive the engine of lubrication entrained in the fuel flow leading to overheating or even catastrophic failure. In any event, a user or operator of the power tool may lack the necessary range of fuel flow adjustment to keep the engine efficiently operating under the full range of temperature, humidity, altitude or other engine operating conditions which may be encountered.
With or without an enclosure or shroud, assembly and proper orientation of these caps is difficult and requires considerable dexterity. If improperly oriented, its tamper proof construction virtually prevents removal without damaging the cap and disturbing the optimum fuel flow setting of the valve.